The two most useful properties of the metal are corrosion resistance and the highest strength-to-density ratio of any metallic element.[6] In its unalloyed condition, titanium is as strong as some steels, but less dense.[7] There are two allotropic forms[8] and five naturally occurring isotopes of this element, 46Ti through 50Ti, with 48Ti being the most abundant (73.8%).[9] Although they have the same number of valence electrons and are in the same group in the periodic table, titanium and zirconium differ in many chemical and physical properties.

Commercial (99.2% pure) grades of titanium have ultimate tensile strength of about 434 MPa (63,000 psi), equal to that of common, low-grade steel alloys, but are less dense. Titanium is 60% denser than aluminium, but more than twice as strong[7] as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys (e.g., Beta C) achieve tensile strengths of over 1400 MPa (200000 psi).[11] However, titanium loses strength when heated above 430 °C (806 °F).[12]

Titanium is not as hard as some grades of heat-treated steel, is non-magnetic and a poor conductor of heat and electricity. Machining requires precautions, as the material might gall if sharp tools and proper cooling methods are not used. Like those made from steel, titanium structures have a fatigue limit which guarantees longevity in some applications.[10] Titanium alloys have lower stiffness than in many other structural materials such as aluminium alloys and carbon fiber.

The metal is a dimorphic allotrope whose hexagonal alpha form changes into a body-centered cubic (lattice) β form at 882 °C (1,620 °F).[12] The specific heat of the alpha form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the β form regardless of temperature.[12] Similar to zirconium and hafnium, an additional omega phase exists, which is thermodynamically stable at high pressures, but is metastable at ambient pressures. This phase is usually hexagonal (ideal) or trigonal (distorted) and can be viewed as being due to a soft longitudinal acoustic phonon of the β phase causing collapse of (111) planes of atoms.[13]

Chemical properties

Like aluminium and magnesium metal surfaces, titanium metal and its alloys oxidize immediately upon exposure to air. Titanium readily reacts with oxygen at 1,200 °C (2,190 °F) in air, and at 610 °C (1,130 °F) in pure oxygen, forming titanium dioxide.[8] It is, however, slow to react with water and air, as it forms a passive and oxide coating that protects the bulk metal from further oxidation.[3] When it first forms, this protective layer is only 1–2 nm thick but continues to slowly grow; reaching a thickness of 25 nm in four years.[15]

Related to its tendency to form a passivating layer, titanium exhibits excellent resistance to corrosion. It is almost as resistant as platinum, capable of withstanding attack by dilute sulfuric and hydrochloric acids as well as chloride solutions, and most organic acids.[4] However, it is attacked by concentrated acids.[16] As indicated by its negative redox potential, titanium is thermodynamically a very reactive metal. One indication is that the metal burns before its melting point is reached. Melting is only possible in an inert atmosphere or in a vacuum. At 550 °C (1,022 °F), it combines with chlorine.[4] It also reacts with the other halogens and absorbs hydrogen.[5]

Titanium is one of the few elements that burns in pure nitrogen gas, reacting at 800 °C (1,470 °F) to form titanium nitride, which causes embrittlement.[17] Because of its high reactivity toward oxygen, nitrogen and some other gases, titanium filaments are applied in titanium sublimation pumps as scavengers for these gases. Such pumps inexpensively and reliably produce extremely low pressures in ultra-high vacuum systems.

Occurrence

Titanium is always bonded to other elements in nature. It is the ninth-most abundant element in Earth's crust (0.63% by mass)[19] and the seventh-most abundant metal. It is present in most igneous rocks and in sediments derived from them (as well as in living things and natural bodies of water).[3][4] Of the 801 types of igneous rocks analyzed by the United States Geological Survey, 784 contained titanium. Its proportion in soils is approximately 0.5 to 1.5%.[19]

It is widely distributed and occurs primarily in the mineralsanatase, brookite, ilmenite, perovskite, rutile and titanite (sphene).[15] Of these minerals, only rutile and ilmenite have economic importance, yet even they are difficult to find in high concentrations. About 6.0 and 0.7 million tonnes of these minerals have been mined in 2011, respectively.[18] Significant titanium-bearing ilmenite deposits exist in western Australia, Canada, China, India, Mozambique, New Zealand, Norway, Ukraine and South Africa.[15] About 186,000 tonnes of titanium metal sponge were produced in 2011, mostly in China (60,000 t), Japan (56,000 t), Russia (40,000 t), United States (32,000 t) and Kazakhstan (20,700 t). Total reserves of titanium are estimated to exceed 600 million tonnes.[18]

The concentration of Ti is about 4 picomolar in the ocean. At 100 °C, the concentration of titanium in water is estimated to be less than 10−7 M at pH 7. The identity of titanium species in aqueous solution remains unknown because of its low solubility and the lack of sensitive spectroscopic methods, although only the 4+ oxidation state is stable in air. No evidence exists for a biological role for titanium, although rare organisms are known to accumulate high concentrations.[20]

Titanium is contained in meteorites and has been detected in the Sun and in M-typestars,[4] which are the coolest type of star, with a surface temperature of 3,200 °C (5,790 °F).[21]Rocks brought back from the Moon during the Apollo 17 mission are composed of 12.1% TiO2.[4] It is also found in coal ash, plants, and even the human body.

Isotopes

Naturally occurring titanium is composed of 5 stable isotopes: 46Ti, 47Ti, 48Ti, 49Ti, and 50Ti, with 48Ti being the most abundant (73.8% natural abundance). Eleven radioisotopes have been characterized, with the most stable being 44Ti with a half-life of 63 years, 45Ti with a half-life of 184.8 minutes, 51Ti with a half-life of 5.76 minutes, and 52Ti with a half-life of 1.7 minutes. All of the remaining radioactive isotopes have half-lives that are less than 33 seconds and the majority of these have half-lives that are less than half a second.[9]

Oxides, sulfides, and alkoxides

The most important oxide is TiO2, which exists in three important polymorphs; anatase, brookite, and rutile. All of these are white diamagnetic solids, although mineral samples can appear dark (see rutile). They adopt polymeric structures in which Ti is surrounded by six oxide ligands that link to other Ti centers.

A variety of reduced oxides of titanium are known. Ti3O5, described as a Ti(IV)-Ti(III) species, is a purple semiconductor produced by reduction of TiO2 with hydrogen at high temperatures,[24] and is used industrially when surfaces need to be vapour-coated with titanium dioxide: it evaporates as pure TiO, whereas TiO2 evaporates as a mixture of oxides and deposits coatings with variable refractive index.[25] Also known is Ti2O3, with the corundum structure, and TiO, with the rock salt structure, although often nonstoichiometric.[26]

The alkoxides of titanium(IV), prepared by reacting TiCl4 with alcohols, are colourless compounds that convert to the dioxide on reaction with water. They are industrially useful for depositing solid TiO2 via the sol-gel process. Titanium isopropoxide is used in the synthesis of chiral organic compounds via the Sharpless epoxidation.

Titanium forms a variety of sulfides, but only TiS2 has attracted significant interest. It adopts a layered structure and was used as a cathode in the development of lithium batteries. Because Ti(IV) is a "hard cation", the sulfides of titanium are unstable and tend to hydrolyze to the oxide with release of hydrogen sulfide.

In the 1950s and 1960s the Soviet Union pioneered the use of titanium in military and submarine applications[38] (Alfa class and Mike class)[41] as part of programs related to the Cold War.[42] Starting in the early 1950s, titanium began to be used extensively for military aviation purposes, particularly in high-performance jets, starting with aircraft such as the F100 Super Sabre and Lockheed A-12 and SR-71.

Throughout the period of the Cold War, titanium was considered a strategic material by the U.S. government, and a large stockpile of titanium sponge was maintained by the Defense National Stockpile Center, which was finally depleted in the 2000s.[45] According to 2006 data, the world's largest producer, Russian-based VSMPO-Avisma, was estimated to account for about 29% of the world market share.[46] As of 2015, titanium sponge metal was produced in six countries: China, Japan, Russia, Kazakhstan, the USA, Ukraine and India. (in order of output).[47][48]

In 2006, the U.S. Defense Advanced Research Projects Agency (DARPA) awarded $5.7 million to a two-company consortium to develop a new process for making titanium metal powder. Under heat and pressure, the powder can be used to create strong, lightweight items ranging from armor plating to components for the aerospace, transport, and chemical processing industries.[49]

Production and fabrication

Titanium (mineral concentrate)

Basic titanium products: plate, tube, rods and powder

The processing of titanium metal occurs in 4 major steps:[50] reduction of titanium ore into "sponge", a porous form; melting of sponge, or sponge plus a master alloy to form an ingot; primary fabrication, where an ingot is converted into general mill products such as billet, bar, plate, sheet, strip, and tube; and secondary fabrication of finished shapes from mill products.

Because it cannot be readily produced by reduction of its dioxide,[10] titanium metal is obtained by reduction of TiCl4 with magnesium metal in the Kroll Process. The complexity of this batch production in the Kroll process explains the relatively high market value of titanium,[51] despite the Kroll process is less expensive than the Hunter process.[38] To produce the TiCl4 required by the Kroll process, the dioxide is subjected to carbothermic reduction in the presence of chlorine. In this process, the chlorine gas is passed over a red-hot mixture of rutile or ilmenite in the presence of carbon. After extensive purification by fractional distillation, the TiCl4 is reduced with 800 °C molten magnesium in an argon atmosphere.[8] Titanium metal can be further purified by the van Arkel–de Boer process, which involves thermal decomposition of titanium tetraiodide.

A more recently developed batch production method, the FFC Cambridge process,[52] converts titanium dioxide powder (a refined form of rutile) as feedstock to make Ti metal, either a powder or sponge. The process involves fewer steps than the Kroll process, and it takes less time.[53] If mixed oxide powders are used, the product is an alloy.

Common titanium alloys are made by reduction. For example, cuprotitanium (rutile with copper added is reduced), ferrocarbon titanium (ilmenite reduced with coke in an electric furnace), and manganotitanium (rutile with manganese or manganese oxides) are reduced.[54]

2 FeTiO3 + 7 Cl2 + 6 C → 2 TiCl4 + 2 FeCl3 + 6 CO (900 °C)

TiCl4 + 2 Mg → 2 MgCl2 + Ti (1100 °C)

About 50 grades of titanium and titanium alloys are designated and currently used, although only a couple of dozen are readily available commercially.[55] The ASTM International recognizes 31 Grades of titanium metal and alloys, of which Grades 1 through 4 are commercially pure (unalloyed). These four are distinguished by their varying degrees of tensile strength, as a function of oxygen content, with Grade 1 being the most ductile (lowest tensile strength with an oxygen content of 0.18%), and Grade 4 the least (highest tensile strength with an oxygen content of 0.40%).[15] The remaining grades are alloys, each designed for specific purposes, be it ductility, strength, hardness, electrical resistivity, creep resistance, resistance to corrosion from specific media, or a combination thereof.[56]

The grades covered by ASTM and other alloys are also produced to meet Aerospace and Military specifications (SAE-AMS, MIL-T), ISO standards, and country-specific specifications, as well as proprietary end-user specifications for aerospace, military, medical, and industrial applications.[57]

Titanium powder is manufactured using a flow production process known as the Armstrong process[58] that is similar to the batch production Hunter process. A stream of titanium tetrachloride gas is added to a stream of molten sodium metal; the products sodium chloride salt and titanium particles can be extracted by filtering out extra amount of sodium. Afterwards, titanium can be separated from the salt by water washing. Both sodium and chlorine in the saltwater byproduct are recycled to generate sodium for re-use and chlorine for the initial titanium tetrachloride production.[59]

In terms of fabrication, all welding of titanium must be done in an inert atmosphere of argon or helium in order to shield it from contamination with atmospheric gases such as oxygen, nitrogen, or hydrogen.[12] Contamination will cause a variety of conditions, such as embrittlement, which will reduce the integrity of the assembly welds and lead to joint failure.

Commercially pure flat product (sheet, plate) can be formed readily, but processing must take into account the fact that the metal has a "memory" and tends to spring back. This is especially true of certain high-strength alloys.[60][61] Titanium cannot be soldered without first pre-plating it in a metal that is solderable.[62] The metal can be machined using the same equipment and via the same processes as stainless steel.[12]

Applications

A titanium cylinder, "Grade 2" quality

Titanium is used in steel as an alloying element (ferro-titanium) to reduce grain size and as a deoxidizer, and in stainless steel to reduce carbon content.[3] Titanium is often alloyed with aluminium (to refine grain size), vanadium, copper (to harden), iron, manganese, molybdenum, and with other metals.[63] Applications for titanium mill products (sheet, plate, bar, wire, forgings, castings) can be found in industrial, aerospace, recreational, and emerging markets. Powdered titanium is used in pyrotechnics as a source of bright-burning particles.

TiO
2 powder is chemically inert, resists fading in sunlight, and is very opaque: this allows it to impart a pure and brilliant white color to the brown or gray chemicals that form the majority of household plastics.[5] In nature, this compound is found in the mineralsanatase, brookite, and rutile.[3] Paint made with titanium dioxide does well in severe temperatures, and stands up to marine environments.[5] Pure titanium dioxide has a very high index of refraction and an optical dispersion higher than diamond.[4] In addition to being a very important pigment, titanium dioxide is also used in sunscreens.[10]

Due to its high corrosion resistance to sea water, titanium is used to make propeller shafts and rigging and in the heat exchangers of desalination plants;[4] in heater-chillers for salt water aquariums, fishing line and leader, and for divers' knives. Titanium is used to manufacture the housings and other components of ocean-deployed surveillance and monitoring devices for scientific and military use. The former Soviet Union developed techniques for making submarines with hulls of titanium alloys.[70] Techniques were developed in the Soviet Union to forge titanium in huge vacuum tubes.[66]

Industrial

Welded titanium pipe and process equipment (heat exchangers, tanks, process vessels, valves) are used in the chemical and petrochemical industries primarily for corrosion resistance. Specific alloys are used in downhole and nickelhydrometallurgy applications due to their high strength (e. g.: titanium Beta C alloy), corrosion resistance, or combination of both. The pulp and paper industry uses titanium in process equipment exposed to corrosive media such as sodium hypochlorite or wet chlorine gas (in the bleachery).[71] Other applications include: ultrasonic welding, wave soldering,[72] and sputtering targets.[73]

Titanium tetrachloride (TiCl4), a colorless liquid, is important as an intermediate in the process of making TiO2 and is also used to produce the Ziegler–Natta catalyst. Titanium tetrachloride is also used to iridize glass and, because it fumes strongly in moist air, it is used to make smoke screens.[10]

Consumer and architectural

Titanium metal is used in automotive applications, particularly in automobile or motorcycle racing, where weight reduction is critical while maintaining high strength and rigidity.[74] The metal is generally too expensive to make it marketable to the general consumer market, other than high-end products, particularly for the racing/performance market. Some late model Corvettes have been available with titanium exhausts,[75] and the new Corvette Z06's LT4 supercharged engine uses lightweight, solid titanium intake valves for greater strength and resistance to heat.[76]

Titanium is used in many sporting goods: tennis rackets, golf clubs, lacrosse stick shafts; cricket, hockey, lacrosse, and football helmet grills; and bicycle frames and components. Although not a mainstream material for bicycle production, titanium bikes have been used by race teams and adventure cyclists.[77] Titanium alloys are also used in spectacle frames.[78] This results in a rather expensive, but highly durable and long lasting frame which is light in weight and causes no skin allergies. Many backpackers use titanium equipment, including cookware, eating utensils, lanterns, and tent stakes.[78] Though slightly more expensive than traditional steel or aluminium alternatives, these titanium products can be significantly lighter without compromising strength. Titanium is also favored for use by farriers, because it is lighter and more durable than steel when formed into horseshoes.[78]

Titanium has occasionally been used in architectural applications: the 40 m (131 foot) memorial to Yuri Gagarin, the first man to travel in space, in Moscow (Lua error in Module:Coordinates at line 668: callParserFunction: function "#coordinates" was not found.), is made of titanium for the metal's attractive color and association with rocketry.[79] The Guggenheim Museum Bilbao and the Cerritos Millennium Library were the first buildings in Europe and North America, respectively, to be sheathed in titanium panels.[67] Other construction uses of titanium sheathing include the Frederic C. Hamilton Building in Denver, Colorado[80] and the 107 m (350 foot) Monument to the Conquerors of Space in Moscow.[81]

Because of its superior strength and light weight when compared to other metals traditionally used in firearms (steel, stainless steel, and aluminium), and advances in metalworking techniques, the use of titanium has become more widespread in the manufacture of firearms. Primary uses include pistol frames and revolver cylinders. For these same reasons, it is also used in the body of laptop computers (for example, in Apple's PowerBook line).[82]

Some upmarket categories of tools made to be lightweight and corrosion-resistant, such as shovels and flashlights, are made of titanium or titanium alloys as well.

Jewelry

Because of its durability, titanium has become more popular for designer jewelry (particularly, titanium rings).[78] Its inertness makes it a good choice for those with allergies or those who will be wearing the jewelry in environments such as swimming pools. Titanium is also alloyed with gold to produce an alloy that can be marketed as 24-carat gold, as the 1% of alloyed Ti is insufficient to require a lesser mark. The resulting alloy is roughly the hardness of 14-carat gold and thus is more durable than a pure 24-carat gold item would be.[83]

Titanium's durability, light weight, dent- and corrosion resistance makes it useful in the production of watch cases.[78] Some artists work with titanium to produce artworks such as sculptures, decorative objects and furniture.[84]

The inertness and ability to be attractively colored makes titanium a popular metal for use in body piercing.[85] Titanium may be anodized to produce various colors, which varies the thickness of the surface oxide layer and causes interference fringes.[86]

Titanium has a minor use in dedicated non-circulating coins and medals. In 1999 Gibraltar released world's first titanium coin for the millennium celebration.[87] The Gold Coast Titans, an Australian rugby league team, award a medal of pure titanium to their player of the year.[88]

Medical

Titanium biocompatibility: Because it is biocompatible (it is non-toxic and is not rejected by the body), titanium has many medical uses, including surgical implements and implants, such as hip balls and sockets (joint replacement) that can stay in place for up to 20 years.[35] The titanium is often alloyed with about 4% aluminium or 6% Al and 4% vanadium.[89]

Titanium has the inherent ability to osseointegrate, enabling use in dental implants that can last for over 30 years. This property is also useful for orthopedic implant applications.[35] These benefit from titanium's lower modulus of elasticity (Young's modulus) to more closely match that of the bone that such devices are intended to repair. As a result, skeletal loads are more evenly shared between bone and implant, leading to a lower incidence of bone degradation due to stress shielding and periprosthetic bone fractures, which occur at the boundaries of orthopedic implants. However, titanium alloys' stiffness is still more than twice that of bone, so adjacent bone bears a greatly reduced load and may deteriorate.[90]

Because titanium is non-ferromagnetic, patients with titanium implants can be safely examined with magnetic resonance imaging (convenient for long-term implants). Preparing titanium for implantation in the body involves subjecting it to a high-temperature plasma arc which removes the surface atoms, exposing fresh titanium that is instantly oxidized.[35]

Nuclear waste storage

Due to its extreme corrosion resistance, titanium containers have been studied for the long-term storage of nuclear waste (containers lasting over 100,000 years are possible under proper manufacturing conditions to reduce defects in the process).[91] A titanium "drip shield" could also be placed over other types of containers to further contain the waste.[92]

Bioremediation

Precautions

Titanium is non-toxic even in large doses and does not play any natural role inside the human body.[21] An estimated quantity of 0.8 milligrams of titanium is ingested by humans each day, but most passes through without being absorbed.[21] It does, however, have a tendency to bio-accumulate in tissues that contain silica. One study indicates a possible connection between titanium and yellow nail syndrome.[94] An unknown mechanism in plants may use titanium to stimulate the production of carbohydrates and encourage growth. This may explain why most plants contain about 1 part per million (ppm) of titanium, food plants have about 2 ppm, and horsetail and nettle contain up to 80 ppm.[21]

As a powder or in the form of metal shavings, titanium metal poses a significant fire hazard and, when heated in air, an explosion hazard.[95] Water and carbon dioxide–based methods to extinguish fires are ineffective on burning titanium; Class D dry powder fire fighting agents must be used instead.[5]

When used in the production or handling of chlorine, care must be taken to use titanium only in locations where it will not be exposed to dry chlorine gas which can result in a titanium/chlorine fire.[96] A fire hazard exists even when titanium is used in wet chlorine due to possible unexpected drying brought about by extreme weather conditions.

Titanium can catch fire when a fresh, non-oxidized surface comes in contact with liquid oxygen.[97] Such surfaces can appear when the oxidized surface is struck with a hard object, or when a mechanical strain causes the emergence of a crack. This poses the possible limitation for its use in liquid oxygen systems, such as those found in the aerospace industry. Due to titanium tubing manufacturing impurities that could cause fires when exposed to oxygen, titanium is prohibited in the construction of gaseous oxygen systems also called aviation breathing oxygen. Steel tubing is used for high pressure systems (3,000 p.s.i.) and aluminum tubing for low pressure systems.